The present invention relates to a control apparatus for an automatic transmission of a vehicle which is configured to control a transmission gear ratio by using a hydraulic pressure of a hydraulic fluid.
A belt-type continuously-variable transmission includes a primary pulley (driving pulley), a secondary pulley (driven pulley), and a belt wound around the primary pulley and the secondary pulley. The primary pulley includes a fixed pulley (sheave) integrally formed with a shaft (primary shaft) serving as a rotation axis, and a movable pulley (slidable sheave) arranged to be moved in an axial direction of the primary shaft, and located at a position to confront the fixed pulley of the primary pulley. The secondary pulley includes a fixed pulley (sheave) integrally formed with a shaft (secondary shaft) serving as a rotation axis, and a movable pulley (sheave) arranged to be moved in an axial direction of the secondary shaft, and located at a position to confront the fixed pulley of the secondary pulley. The belt is contacted, under pressure, with a V-shaped groove defined by the fixed pulley and the movable pulley of the primary pulley, and with a V-shaped groove defined by the fixed pulley and the movable pulley of the secondary pulley, and accordingly power transmission is performed by the compressive contact between the belt and each of the primary and the secondary pulley.
The movable pulley of the primary pulley is arranged to move axially toward or move axially apart from the fixed pulley of the primary pulley by regulating a hydraulic pressure within a hydraulic chamber formed on a back surface (or rear surface) of the movable pulley of the primary pulley. The movable pulley of the secondary pulley is arranged to move axially toward or move axially apart from the fixed pulley of the secondary pulley by regulating a hydraulic pressure within a hydraulic chamber formed on a back surface (or rear surface) of the movable pulley of the secondary pulley. This movements (stroke displacement) of the movable pulleys varies groove widths of the V-shaped grooves respectively, and adjust effective radiuses of rotation of the primary and secondary pulleys. Consequently, it is possible to vary a power transmission ratio from the primary pulley to the secondary pulley in a stepless manner.
In a case in which a transmission gear ratio (pulley ratio) is increased (that is, in a case in which the transmission gear ratio is shifted to a low side), the hydraulic pressure (an actual secondary pressure) of the hydraulic chamber of the secondary pulley is increased to move the movable pulley of the secondary pulley toward the fixed pulley of the secondary pulley. Consequently, the groove width of the V-shaped groove of the secondary pulley is decreased, and the effective radius of the rotation of the secondary pulley is increased. In this state, the radius of the rotation of the primary pulley is decreased as the radius of the rotation of the secondary pulley is increased because length of the belt does not vary, so that it is possible to increase the pulley ratio.
In the conventional belt-type continuously-variable transmission, at the shift, a target pulley ratio is first set in accordance with a vehicle speed and a throttle opening (TVO). A target time constant (a constant which correlates to a shift speed, and which is appropriately adjusted) is set at each of an upshift, a downshift, and a depression-induced downshift (downshift accompanying a driver's depression). The target pulley ratio is delayed by the target time constant at the first order, and is set to a transient target pulley ratio (the target pulley ratio at a transient of the shift). The control apparatus performs a feedback control based on the target pulley ratio and the actual pulley ratio. In this way, the feed back control is performed so that the target pulley ratio is not set directly to a target value, and that the target pulley ratio with the first-order delay is set to the target value. When the actual pulley ratio at a start of the shift is varied to the target pulley ratio, the actual pulley ratio is substantially linearly varied in the range of the shift speed at a normal control. Accordingly, the target value at the feedback control is linearly varied in accordance with the variation in the actual pulley ratio, and it is possible to compensate following ability of the actual pulley ratio with respect to the target pulley ratio, and to perform the smooth shift corresponding to the variation of the actual pulley ratio.